Slope time can either be upslope or down slope as shown in the diagram below. Both are considered distinct segments of but part of the total weld time.
Upslope is a term to describe the beginning a weld cycle when the current is gradually increased from some lower value to a higher value over a period of time usually 3-10 cycles in standard AC welding. At the end of the upslope period then normally the full desired weld current is applied for the desired length of time.
Upslope is shown in the first section of the red current line - as it increases. This period is often used on galvanized materials to allow the zinc to melt and be pushed out of the joint. In other cases it is a chance for the components to anneal or relax and get all material into full contact before full power is applied.
Downslope is a gradual reduction in current as shown on the red line to cool the weld joint slowly. In this case it helps maintain some heat to extent the time for the forging action to occur.
The weld time is the total time of the red curve which includes the upslope, weld current, downslope and post heat.
Reference: Resistance Welding Manual, Chapter 20